North Korea has detained a 24-year-old American man for improper behaviour while he was being processed to enter the country as a tourist, state media reported.

The official Korean Central News Agency identified the man as Miller Matthew Todd - possibly putting his surname first - and said he entered the country on April 10 with a tourist visa, but tore it up and shouted that he wanted to seek asylum. The brief report said he chose the North "as a shelter".

It said he was detained for "gross violation" of North Korea's legal order and was being investigated.

In Washington, US State Department spokeswoman Jen Psaki told reporters that the US is aware of the report, but she did not confirm an American was being held. She said the department is in touch with the Swedish Embassy which handles consular cases for the US because Washington does not have diplomatic relations with Pyongyang.

"There is no greater priority to us than the welfare and safety of US citizens. We don't have additional information to share at this time," Ms Psaki said.

A large number of foreign tourists were in North Korea in mid-April to see events held for the anniversary of national founder Kim Il Sung's April 15 birthday. One of the main events, the annual Pyongyang marathon, was opened to foreign amateur runners for the first time this year and drew well over 100 tourists.

North Korea has been trying to boost tourism recently to generate income. Earlier this year, it opened a new luxury ski resort and it is planning to develop special zones for tourism, mostly from China, across the country.

But the North also continues to be highly sensitive about the activities and conduct of foreigners who are allowed in.

North Korea has been holding a Korean-American missionary, Kenneth Bae, since November 2012. Bae was sentenced to 15 years of hard labour for what the North has claimed were hostile acts against the state. In March, it deported an Australian missionary detained for spreading Christianity in the country after he apologised for anti-state religious acts and requested forgiveness.

The announcement came as President Barack Obama was visiting rival South Korea. Mr Obama warned North Korea that it would face tougher sanctions if it follows through with threats to conduct a fourth nuclear test.